The present invention relates to an improved method for the production of N-vinyl-2-pyrrolidone (NVP). The method uses hydroxy end-capped ether oligomers having a molecular weight less than 1000 or linear diols having more than 4 carbon atoms as co-catalysts for increasing the activity of alkali metal salt catalysts in order to accelerate the vinylation of 2-pyrrolidone with acetylene and obtain a higher yield of N-vinyl-2-pyrrolidone (NVP).
NVP is a valuable and useful compound. Due to its unique physical properties such as water solubility, high polarity, nontoxicity, chemical stability, and cation activity, it has been widely applied in the manufacture of adhesives, paints, textiles, foods and personal medicines. The homopolymers or copolymers thereof have improved film strength, dye compatability, rigity and adhesion.
A conventional method for making NVP is the "Reppe Reaction" which uses 2-pyrrolidone and acetylene as raw materials, and alkali metal salts as catalysts(Brit. Patent No. 1,045,627, U.S.S.R. U.S. Pat. No. 198,339, and Japanese Patent No. 71,09,458). According to the method, NVP is prepared by vinylation in an autoclave at 170.degree. C. and under low pressure or atmospheric pressure. The Reppe method has the disadvantages of low yield and the formation of nonvolatile polymer residues, making the separation and recovery of the desired product difficult. In order to eliminate the disadvantages, BASF and GAF company adopt a two-stage method to produce NVP (CHEM SYSTEM Report,89S9). According to their method, raw material, 2-pyrrolidone (2-p) is first reacted with KOH serving as precursor catalyst to produce a potassium salt catalyst and water. To avoid production of any by-product, potassium 4-amino-butyrate, water must be removed in this stage, and thus the reaction must be operated under vacuum and purged by N.sub.2 in batch reactors. After the water is removed completely, the potassium salt catalyst is then reacted with acetylene by vinylation to produce NVP. The whole reaction is depicted in the following reaction scheme: ##STR1##
The disadvantage of this method is that the formed water must be removed continuously, otherwise in step (1) the reaction will readily reach an equilibrium state and will not proceed to the right side. Another disadvantage is that the presence of even a trace of water will adversely affect the activity of the potassium salt catalysts, and thus result in low conversion. Generally according to this method, the conversion of 2-pyrrolidone is between 47% and 62%, and the selectivity is about 90%. Moreover, in the case where the conversion is increased, non-volatile polymer residues are likely to be produced, resulting in difficulty of separation.
To increase the yield of NVP and decrease the formation of polymer residues, in U.S. Pat. No.4,410,726, it is disclosed that polyoxyalkylenes (e.g. crown ether) having a molecular weight less than 2000 serving as co-catalysts are added in the vinylation reaction to increase the selectivity of NVP and the reaction rate. The yield is elevated to 70% and the selectivity is 94% under proper reaction conditions. However, a vast amount of polyoxyalkylene, usually 2 to 4 times the amount of KOH catalyst used has to be added according to this method. This kind of co-catalyst is not only very expensive but also unable to completely inhibit the formation of the polymer residues. The yield of polymer residues remains 4.5% to 6.5%, and still causes difficulty in separation during the subsequent processes. In addition, the catalyst described in U.S. Pat. No. 4,410,726 tends to be affected by water, resulting in a low NVP yield. The yield is only 70% even when the co-catalyst is added. The unavoidable polymer residues necessitate two distillation towers to separate the NVP from the raw material, 2-pyrrolidone and by-products.